Process for manufacturing a composite polymeric circuit protection device

ABSTRACT

A process for manufacturing a composite polymeric circuit protection device in which a polymeric assembly is provided and is then subdivided into individual devices. The assembly is made by providing first and second laminates, each of which includes a laminar polymer element having at least one conductive surface, providing a pattern on at least one of the conductive surfaces on one laminate, securing the laminates in a stack in a desired configuration, at least one conductive surface of at least one of the laminates forming an external conductive surface of the stack, and making a plurality of electrical connections between a conductive surface of the first laminate and a conductive surface of the second laminate. The laminar polymer elements may be PTC conductive polymer compositions, so that the individual devices made by the process exhibit PTC behavior.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of commonly assigned Application Ser.No. 09/395,869, filed Sep. 14, 1999 now U.S. Pat. No. 6,640,420 thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrical devices and assemblies and methodsof making such devices and assemblies.

2. Introduction to the Invention

Circuit protection devices comprising a conductive polymer compositionhaving a positive temperature coefficient (PTC) are well-known. Suchdevices which are intended for surface mounting onto a substrate, e.g. aprinted circuit board, are disclosed in U.S. Pat. Nos. 5,831,510 (Zhanget al), 5,852,397 (Chan et al), and 5,864,281 (Zhang et al),International Publications Nos. 94/01876 (Raychem Corporation) and95/08176 (Raychem Corporation), and copending, commonly assignedApplication Ser. No. 09/181,028 (Graves et al, filed Oct. 27, 1998), thedisclosures of which are incorporated herein by reference. Such circuitprotection devices generally comprise first and second laminarelectrodes; a laminar PTC resistive element sandwiched between theelectrodes; a third (residual) laminar conductive member which issecured to the same face of the PTC element as the second electrode butis separated therefrom; and a cross-conductor which passes through anaperture in the PTC element and connects the third conductive member andthe first electrode. This permits connection to both electrodes from thesame side of the device, so that the device can be connected flat on aprinted circuit board, with the first electrode on top, without any needfor leads. The resistive element preferably comprises a laminar elementcomposed of a PTC conductive polymer. Preferably the device comprises anadditional conductive member and an additional cross-conductor, so thatthe device is symmetrical and can be placed either way up on a circuitboard.

When two of these devices are physically secured together in a stackedconfiguration, a composite device can be formed. Such composite deviceshave the same small “foot-print” on the board, i.e. occupy a small area,as a single device, but they have a lower resistance than can beconveniently produced by using a single device. In addition, the powerdissipation of such a composite device is not substantially differentfrom the power dissipation of one of the devices alone. As a result, thecomposite device has a lower resistance for a given hold current where“hold current” is the largest current which can be passed through adevice without causing it to trip.

SUMMARY OF THE INVENTION

As described in copending, commonly assigned U.S. patent applicationSer. No. 09/060,278 (Chiang et al, filed Apr. 14, 1998), now U.S. Pat.No. 6,606,023 (issued Aug. 12, 2003). the disclosure of which isincorporated herein by reference, composite devices can be prepared bysorting individual devices and then assembling the sorted devices intocomposite devices. Such a process can be tedious, as it may require thatthe resistance of each individual device be read. We have now found, inaccordance with the present invention, that it is possible to prepare amultilayer assembly from which individual composite devices can bedivided. Such an assembly allows preparation of a large number ofcomposite devices simultaneously. Furthermore, because the processdescribed herein allows the patterning of individual layers of theassembly before or after fabrication into the assembly, a variety ofdifferent devices can be prepared from the same starting layers. Inaddition, the composition of the layers can be easily varied, allowingthe simple build-up of devices with combined functionality. Variousinterconnection schemes between layers can be simply implemented, anddevices with multiple external electrical contacts can be made withoutchanging the basic manufacturing process. All of these further add tothe broad range of different devices which can be inexpensivelymass-produced by the process disclosed herein.

This invention provides methods and processes for which variousoperative steps can be carried out on an assembly which will yield aplurality of devices when subdivided into composite devices bysubdividing along both x- and y-directions (where x and y correspond todirections in the plane of the laminar PTC elements). The ability toprepare devices in this way is a significant improvement over othermethods, for example that described in U.S. application Ser. No.09/060,278 (now U.S. Pat. No. 6,606,023), because for this invention,individual devices do not need to be individually assembled, increasingthe efficiency and therefore reducing the cost of the manufacturingprocess. Finally, the method of combiining layers of materials to formcomposite devices disclosed herein allows an extremely simple yetadaptable method for forming a variety of devices without the necessityof changing the basic manufacturing process.

In a first aspect this invention provides a process for manufacturing acomposite polymeric circuit protection device, said process comprising

-   -   (1) providing a polymeric assembly comprising        -   (a) providing first and second laminates, each of which            comprises a laminar polymer element having at least one            conductive surface,        -   (b) providing a pattern of conductive material on at least            one of the conductive surfaces on one laminate;        -   (c) securing the laminates in a stack in a desired            configuration, at least one conductive surface of at least            one of the laminates comprising an external conductive            surface of the stack, and        -   (d) making a plurality of electrical connections between a            conductive surface of the first laminate and a conductive            surface of the second laminate; and    -   (2) subdividing the stack into individual devices each of which        comprises at least one electrical connection.

In a second aspect this invention provides a polymeric assemblycomprising:

-   -   (a) a first laminate comprising a laminar polymer element having        at least one conductive surface having a pattern;    -   (b) a second laminate comprising a laminar polymer element        having at least one conductive surface having a pattern, said        second laminate being secured to the first laminate in a stack        so that the stack has first and second external conductive        surfaces; and (c) a plurality of transverse conductive members        which run through the first and second laminates between the        first and second external conductive surfaces.

Using either the process or the assembly of the invention, devices canbe made by creating electrode precursors in the form of conductivesurfaces of appropriate shapes upon resistive elements which are largerthan the desired final shape, forming a stack of a plurality ofresistive elements which is also larger than the desired final shape,and then subdividing the stack into individual devices. Electrodes ofappropriate shapes can be made by removing unwanted portions of any one,or any combination, of the conductive surfaces. The removal can beaccomplished by milling, stamping, or etching, for example.Alternatively, the electrode precursors can be formed by patterningconductive material onto any one or any combination of the PTC resistiveelement surfaces by chemical vapor deposition, electrodeposition,sputtering, etc. Conductive material may also be applied to the faces ofthe PTC resistive elements by use of an adhesive or tie layer.Electrical interconnection between a desired combination of theconductive surfaces of the plurality of resistive elements can beaccomplished before the stack has been subdivided into individualdevices. Alternatively, some or all of the electrical connectionsbetween desired electrodes or contact points can be made after the stackhas been subdivided into composite devices. The electricalinterconnection can be designed so that connection is made between someof the conductive surfaces of the stack or the electrodes of the device,but not all.

Thus in a third aspect, this invention provides a composite device,which can be made, for example, using the process of the first aspect ofthe invention or the assembly of the second aspect, comprising

-   -   (1) first and second external laminar electrodes,    -   (2) third and fourth internal laminar electrodes,    -   (3) first and second laminar PTC resistive elements, each of        which (i) exhibits PTC behavior, and (ii) comprises a laminar        element composed of a PTC conductive polymer,        -   said first resistive element having a first face to which            the first external electrode is secured and an opposite            second face to which the third internal electrode is            secured, and said second resistive element having a first            face to which the second external electrode is secured and            an opposite second face to which the fourth internal            electrode is secured,    -   (4) a fifth external laminar conductive member which is (i)        secured to the first face of the first PTC resistive element,        and (ii) is spaced apart from the first external electrode,    -   (5) a sixth external laminar conductive member which (i) is        secured to the first face of the second PTC resistive element,        and (ii) is spaced apart from the second external electrode,    -   (6) a seventh internal laminar conductive member which (i) is        secured to the second face of the first PTC resistive element,        and (ii) is spaced apart from the third internal electrode,    -   (7) an eighth internal laminar conductive member which (i) is        secured to the first face of the second PTC resistive element,        and (ii) is spaced apart from the fourth internal electrode,    -   (8) a first aperture which runs between the first external        electrode of the first laminar PTC element and the second        external electrode of the second laminar PTC element,    -   (9) a second aperture which runs between the fifth external        laminar conductive member of the first laminar PTC element and        the sixth external laminar conductive member of the second        laminar PTC element,    -   (10) a first transverse conductive member which        -   (a) lies within the first aperture,        -   (b) runs between the first external electrode of the first            laminar PTC element and the second external electrode of the            second laminar PTC element,        -   (c) is secured to the first PTC element, the second PTC            element and the third laminar element, and        -   (d) is physically and electrically connected to the first            external laminar electrode, the seventh internal laminar            conductive member, the eighth internal laminar conductive            member, and the second external laminar electrode, but is            not connected to the third or the fourth internal electrode,            and    -   (11) a second transverse conductive member which        -   (a) lies within the second aperture,        -   (b) runs between the fifth external laminar conductive            member and the sixth external laminar conductive member,        -   (c) is secured to the first PTC element, the second PTC            element and the third laminar polymer layer, and        -   (d) is physically and electrically connected to the fifth            external laminar conductive member, the third internal            electrode, the fourth internal electrode, and the sixth            external laminar conductive member, but is not connected to            the first or second external electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying drawings, in which

FIG. 1 is a perspective view of a section of a stack formed in the firstaspect of the invention, which can be subdivided into a plurality ofindividual composite devices;

FIG. 2 is an exploded view of a stack which has been patterned on theinternal conductive surfaces;

FIG. 3 is a plan view of the top of a section of a stack;

FIG. 4 is a cross-sectional view of a section of a stack along lineIV-IV in FIG. 3;

FIG. 5 is a perspective view of a composite device of the invention;

FIG. 6 is a cross-sectional view of a composite device mounted on aprinted circuit board parallel to the board;

FIG. 7 is a plan view of composite devices further illustrated in FIGS.8, 9, 10, 11, 14, and 21;

FIGS. 8, 9, and 10 are cross-sectional views along line VIII-VIII inFIG. 7 of composite devices with two elements connected in parallel;

FIG. 11 is a cross-sectional view along line VIII-VIII in FIG. 7 of acomposite device with three elements connected in parallel;

FIG. 12 is a plan view of another composite device with two elementsconnected in parallel but with no residual conductive members asillustrated in FIG. 13;

FIG. 13 is a cross-sectional view along line XIII-XIII in FIG. 12;

FIG. 14 is a cross-sectional view along line VIII-VIII of FIG. 7 ofanother composite device with two elements connected in parallel;

FIG. 15 is a cross-sectional view of a composite device with twoelements connected in series;

FIG. 16 is a plan view of a composite device with more than two externalelectrical connection points;

FIG. 17 is an electrical diagram of the interconnection scheme of theindividual devices connected together to form the composite devices ofFIGS. 16 and 18 to 20.

FIGS. 18, 19, and 20 are cross-sectional views along lines XVIII-XVIII,XIX-XIX, and XX-XX, respectively, in FIG. 16;

FIG. 21 is a cross-sectional view of a composite device with-twoexternal electrodes and one internal electrode;

FIG. 22 is a plan view of a composite device with multiple electricalconnections between layers of a composite device; and

FIG. 23 is a cross-sectional view along line XXIII-XXIII in FIG. 22.

DETAILED DESCRIPTION OF THE INVENTION

As described and claimed below, and as illustrated in the Figures, thisinvention incorporates a number of features. Where such a feature isdisclosed in a certain context or as part of a particular combination,it can also be used in other contexts and in other combinations,including combinations of any number of such features.

PTC and Resistive Elements

Assemblies and devices of the invention generally comprise at least onelaminar polymer element or resistive element which comprises a PTCcomposition which exhibits positive temperature coefficient (PTC)behavior, i.e. it shows a sharp increase in resistivity with temperatureover a relatively small temperature range. The term “PTC” is used tomean a composition or device that has an R₁₄ value of at least 2.5and/or an R₁₀₀ value of at least 10, and it is preferred that thecomposition or device should have an R₃₀ value of at least 6, where R₁₄is the ratio of the resistivities at the end and the beginning of a 14°C. range, R₁₀₀ is the ratio of the resistivities at the end and thebeginning of a 100° C. range, and R₃₀ is the ratio of the resistivitiesat the end and the beginning of a 30° C. range.

The PTC compositions used in the present invention are preferablyconductive polymers which comprise a crystalline polymer component and,dispersed in the polymer component, a particulate filler component whichcomprises a conductive filler, e.g. carbon black or a metal. The fillercomponent may also contain a non-conductive filler, which changes notonly the electrical properties of the conductive polymer but also itsphysical properties. The composition can also contain one or more othercomponents, e.g. an antioxidant, crosslinking agent, coupling agent,flame retardant, or elastomer. The PTC composition preferably has aresistivity at 23° C. of less than 50 ohm-cm, particularly less than 10ohm-cm, especially less than 5 ohm-cm. Suitable conductive polymers foruse in this invention are disclosed for example in U.S. Pat. Nos.4,237,441 (van Konynenburg et al), 4,304,987 (van Konynenburg),4,514,620 (Cheng et al), 4,534,889 (van Konynenburg et al), 4,545,926(Fouts et al), 4,724,417 (Au et al),. 4,774,024 (Deep et al), 4,935,156(van Konynenburg et al), 5,049,850 (Evans et al), 5,378,407 (Chandler etal), 5,451,919 (Chu et al), 5,582,770 (Chu et al), 5,747,147 (Wartenberget al), and 5,801,612 (Chandler et al), and U.S. Patent Application No.09/364,504 (Isozaki et al, filed Jul. 30, 1999), now U.S. Pat. No.6,358.438 (issued Mar. 19, 2002). The disclosure of each of thesepatents and applications is incorporated herein by reference.

Alternatively, the PTC composition can be a ceramic material.

Laminar Elements

Devices of the invention preferably comprise PTC resistive elementswhich are laminar elements, and can be composed of one or moreconductive polymer members, at least one of which is composed of a PTCmaterial. When there is more than one conductive polymer member, thecurrent preferably flows sequentially through the differentcompositions, as for example when each composition is in the form of alayer which extends across the whole device. When there is a single PTCcomposition, and the desired thickness of the PTC element is greaterthan that which can conveniently be prepared in a single step, a PTCelement of the desired thickness can conveniently be prepared by joiningtogether, e.g. laminating by means of heat and pressure, two or morelayers, e.g. melt-extruded layers, of the PTC composition. When there ismore than one PTC composition, the PTC element will usually be preparedby joining together, e.g. laminating by means of heat and pressure,elements of the different compositions.

Assemblies of the invention comprise first and second laminates, and maycomprise additional laminates. The first and second laminates eachcomprise a laminar polymer element having at least one conductivesurface, e.g. in the form of a metal foil electrode as described below.In this specification, each laminate is referred to as a layer. Thelaminar elements of the first and second laminates may comprise PTCcompositions which are the same, or the layers may comprise differentPTC compositions. For example, PTC compositions of differentresistivities may be used, and an interconnection scheme devised so thatone layer can act as a heater, and a second layer can act as anovercurrent protection device. The layers may also comprise PTCcompositions of different switching temperatures (i.e. the temperatureat which the device switches from a low resistance to a high resistancestate). For example, such a device may be useful for creating atwo-tiered PTC temperature sensor, with one layer being the mostsensitive for a lower temperature range, and the second layer being themost sensitive for a higher temperature range. Furthermore, one or moreof the laminates may comprise a zero temperature coefficient ofresistance (ZTC) composition or a negative temperature coefficient ofresistance (NTC) composition.

It is not necessary that each of the laminates comprise a conductivelayer. For example, other compositions which can be used for a laminarelement in the composite devices comprise a dielectric material such aspolyester or a filled dielectric material such as FR4 epoxy. This canfunction as an insulating layer which provides extra rigidity to thedevice, or the material can be chosen to assist in the mounting andpackaging of the device. In addition, a laminar element can comprise acomposition which has a relatively high thermal conductivity to aid inthe thermal transfer between layers of the composite, or between asubstrate and a layer of the composite for a surface mounted device.Conversely, a laminar element can comprise a composition which has arelatively low thermal conductivity to act as a thermal insulatorbetween layers or between a layer and the substrate. When it is desiredthat the device have the capability to respond to an overvoltage, alayer of the composite can comprise a material which is normallyinsulating but becomes conducting when a certain voltage threshold levelis reached. Such compositions include varistor particles dispersed in apolymeric matrix. Other compositions which may be useful for variousembodiments of this invention include flame retardant materials,intumescants, and microwave absorbing materials to allow the heating ofthe device using radiation of a specific frequency range.

The thicknesses of the laminar elements comprising the assembly used toprepare a composite device may be different. For example, a very thinlaminar element may be used as one layer to provide an extremely lowresistance, and a thicker laminar element may be used as a second layerto provide mechanical strength.

Electrodes and Conductive Surfaces

Particularly useful devices made by the process of the inventioncomprise at least two metal foil electrodes, with polymer elementssandwiched between them. An especially useful device will comprise astack comprising n polymeric PTC elements, each having two metal foilelectrodes, and (n−1) adhesive layers sandwiched between the PTCelements in an alternating pattern to form a composite device, with thePTC elements comprising the top and bottom components of the stack. Thisdevice will have the electrodes electrically connected such that the PTCelements will be connected in parallel, resulting in a composite devicewhich has a low resistance at 20° C., generally less than 10 ohms,preferably less than 5 ohms, more preferably less than 1 ohm,particularly less than 0.5 ohm, with yet lower resistance beingpossible, e.g. less than 0.05 ohm. Particularly suitable foil electrodesare microrough metal foil electrodes, in particular as disclosed in U.S.Pat. Nos. 4,689,475 (Matthiesen) and 4,800,253 (Kleiner et al), and incopending, commonly assigned U.S. application Ser. No. 08/816,471(Chandler et al, filed Mar. 13, 1997), now U.S. Pat. No. 6,570,483(issued May 27, 2003), the disclosure of each of which is incorporatedherein by reference. The electrodes can be modified so as to producedesired thermal effects and so as to provide electrical contact pointsfor various interconnection points between the layers of the compositedevice to give the desired functionality, and to provide electricalcontact points for mounting the device onto printed circuit boards,sockets, clips, or other suitable applications. Examples of compositedevices which incorporate multiple internal and external contact pointsare illustrated in FIGS. 16 to 20, 22, and 23.

Similar types of metal foil can be used to form the conductive surfacesof the laminates in the polymeric assembly. Alternatively, theconductive surface may be formed from a conductive ink, a sputtered orotherwise applied metal layer, a metal mesh, or another suitable layer.Particularly preferred conductive surfaces are those which can beetched, e.g. for patterning, and/or soldered easily. The conductivesurface of the laminates has a resistivity at 25° C. which is at least100 times lower than the resistivity at 25° C. of the polymer element towhich it is attached.

The patterns may be the same on both sides of a given laminate, or theymay be different. Additional patterns may be created at any point in theprocess, e.g. on an external conductive surface of the laminate once astacked assembly is formed, or as attachments are made to internalconductive surfaces before the stacked assembly is completed.

Apertures and Cross-Conductors

The term “aperture” is used herein to denote an opening which, whenviewed at right angles to the plane of the device,

(a) has a closed cross-section, e.g. a circle, an oval, or a generallypolygonal shape, or

(b) has a reentrant cross-section, the term “reentrant cross-section”being used to denote an open cross-section which (i) has a depth atleast 0.15 times, preferably at least 0.5 times, particularly at least1.2 times, the maximum width of the cross-section, e.g. a quarter circleor a half circle or an open-ended slot, and/or (ii) has at least onepart where the opposite edges of the cross-section are parallel to eachother.

Since the invention involves assemblies which can be divided into aplurality of electrical devices, the apertures will normally be ofclosed cross-section, but if one or more of the lines of division passesthrough an aperture of closed cross-section, then the apertures in theresulting devices will have open cross-sections. Although for someembodiments, it is desirable that an open cross-section is a reentrantcross-section as defined above, in order to ensure that thecross-conductor, which passes through the aperture, is not damaged ordislodged during installation or use of the device, for otherembodiments, it is preferred that the cross-conductor is a plating onone transverse flat face of the device. In order to produce such adevice, it is preferred that the assembly, which is to be divided into aplurality of devices, has a plurality of elongate rectangular apertures,e.g. slots, each with metal plating thereon. The assembly is thendivided so that each plated aperture provides a flat transverseconductive member on a number of devices.

The apertures in the assembly may be of different sizes and/or shapes toaccommodate device configurations and current-carrying capabilities.

The aperture can be a circular hole, and this is satisfactory in manycases. However, if the assembly includes apertures which are traversedby at least one line of division, elongate apertures may be preferredbecause they require less accuracy in the lines of division.

When the aperture is not traversed by a line of division, it can be assmall as is convenient for a cross-conductor having the necessarycurrent-carrying capacity. Generally a single cross-conductor is allthat is needed to make an electrical connection to the first electrodeto the opposite side of the device. However, two or morecross-conductors can be used to make the same connection. The number andsize of the cross-conductors, and, therefore, their thermal capacity,can have an effect on the rate at which a circuit protection device willtrip. Generally the apertures and cross-conductors can extend throughoutall layers of the assembly. Alternatively, apertures andcross-conductors can extend through only some layers of the assembly toform devices of different functionality.

The aperture can be formed before the cross-conductor is put into place,or the formation of the aperture and the placing of the cross-conductorcan be carried out simultaneously. A preferred procedure is to form anaperture, e.g. by drilling, slicing, routing or any other appropriatetechnique, and then to plate or otherwise coat or fill the interiorsurface of the aperture. The plating can be effected by electrolessplating, or electrolytic plating, or by a combination of both. Theplating can be a single layer or multiple layers, and can be composed ofa single metal or a mixture of metals, in particular a solder. Theplating will often be formed on other exposed conductive surfaces of theassembly. If such plating is not desired, then the other exposedconductive surfaces can be masked or otherwise desensitized, orundesired plating can be selectively removed. The invention includes thepossibility that the plating will produce not only the cross-conductorbut also at least part of the laminar conductive members in the device.

The plating techniques which are used for making conductive vias throughinsulating circuit boards can be used in the present invention.

Another technique for providing the cross-conductors is to place amoldable or liquid conductive composition in preformed apertures, and ifdesired or necessary, to treat the composition, while it is in theapertures, so as to produce cross-conductors of desired properties. Thecomposition can be supplied selectively to the apertures, e.g. by meansof a screen, or to the whole assembly, if desired after pretreating atleast some of the assembly so that the composition does not stick to it.For example, a molten conductive composition, e.g. solder, could be usedin this way, if desired, using wave soldering techniques.

The cross-conductor can also be provided by a preformed member, e.g. ametal rod or tube such as a rivet. When such a preformed member is used,it can create the aperture as it is put in place in the device.

The cross-conductor can partially or completely fill the aperture. Whenthe aperture is partially filled, it can be further filled (includingcompletely filled) during the process in which the device is connectedto other electrical components, particularly by a soldering process.This can be encouraged by providing additional solder in and around theaperture, especially by including a plating of solder in and around theaperture. Normally at least a part of the cross-conductor will be put inplace before the device is connected to other electrical components.However, the invention includes the possibility that the cross-conductoris formed during such a connection process, as for example by thecapillary action of solder during a soldering process.

A cross-conductor may be designed such that it electrically connectssome of the layers, i.e. some of the laminates, together, but not all.Such cross-conductors are depicted in FIG. 15. A method for making sucha cross-conductor would comprise forming an aperture which is largerthan the desired size for the cross-conductor, filling the aperture withan insulating substance, forming an inner aperture within the insulatingsubstance, and plating the inner aperture to render it conductive. Thismethod insulates the internal electrodes from the cross-conductor, yetallows the external electrodes to be electrically connected.

Connectors Which are not Cross-Conductors

The electrical connection(s) between the electrode(s) and any residualmember(s) on the various faces of the PTC resistive elements ispreferably through a cross-conductor as described above. However, it canbe of any kind, for example a connector which will remain in place evenif it is not bonded to the other parts of the device, for example aU-shaped member which extends around the ends of a layer or combinationof layers of the device.

Residual Laminar Conductive Members

A preferred embodiment of a device of the invention comprises anadditional (residual) conductive member which is secured to the sameface of the PTC element as the second electrode, but is separatedtherefrom. This residual laminar conductive member which, with the crossconductor or other connector, can be present to provide an electricalpath to other electrodes: is formed by removing part of a laminarconductive member, the remainder of the laminar conductive member thenbeing an electrode. Residual laminar conductive members may be presenton both internal and external faces of the laminar elements. The shapeof the residual laminar conductive member, and the shape of the gapbetween the residual member and an electrode, can be varied to suit thedesired characteristics of the device and for ease of manufacture. Theresidual conductive member is conveniently a small rectangle at one endof a rectangular device, separated from an electrode by a rectangulargap. Alternatively, the residual member can be an island separated fromthe electrode by a gap of closed cross-section. Devices can also bedesigned without a residual laminar conductive member, as shown in FIGS.12 and 13.

Additional Laminar Elements

The first and second laminar PTC resistive elements of the device or thefirst and second laminates of the assembly can be physically securedtogether in a stack, using a third laminar element between them. Thethird laminar element may comprise an electrically nonconductiveadhesive, e.g. a hot-melt adhesive or a curable bonding material, towhich fillers can be added to achieve particular thermal or mechanicalproperties. The third laminar element can also comprise curablemonomeric organic or inorganic systems, such as epoxies, acrylates,allyls, urethanes, phenolics, esters, alkyds, etc. If it is desired thatthe laminar element act as an electrical insulator, then it is preferredthat the resistivity is at least 10⁶ ohm-cm, particularly at least 10⁹ohm-cm. For some embodiments it may be desired that the third laminarelement comprise a conductive material. For these embodiments, the thirdlaminar element-serves to connect the layers together electrically aswell as physically. A composite device configuration incorporating aconductive third laminar element is illustrated in FIG. 15. For otherembodiments, it may be desired that the third element comprise aconductive material which is electrically conductive in only onedirection (see FIG. 14). The third laminar element can also provideother functions, e.g. a thermally conductive layer to facilitate thermaltransfer between the layers of the composite device.

Alternatively, devices can be designed which do not include a separatelaminar layer to secure the elements of the device together. Forexample, a device can be made which is similar to that depicted in FIG.8, with the laminar element 26 eliminated. The cross-conductors 32 and52 can be relied upon to both electrically connect the layers togetherin parallel, and to physically secure the layers together. FIG. 21depicts another example where the composite device does not require aseparate laminar element between the layers.

Devices

In a simple device, as shown in FIG. 5 there are two externalelectrodes, two internal electrodes, two cross-conductors or otherconnectors, and four residual conductive members. This configuration isuseful since the device will then be symmetrical top to bottom, allowingfor ease of installation by automated equipment or otherwise.

Particularly preferred circuit protection devices of the invention havea resistance at 23° C. of less than 1 ohm, preferably less than 0.5 ohm,particularly less than 0.3 ohm, especially less than 0.1 ohm, andcomprise first and second laminar PTC resistive elements, each of which(a) is composed of a conductive polymer which has a resistivity at 23°C. of less than 50 ohm-cm, preferably less than 10 ohm-cm, particularlyless than 5 ohm-cm, and which exhibits PTC behavior, and (b) has a firstface and a second face. A first external metal foil electrode contactsthe first face of the first PTC element and a second external metal foilelectrode contacts the first face of the second PTC element. Third andfourth internal metal foil electrodes contact the second faces of thefirst and second PTC elements, respectively. The device preferably has afifth and sixth residual external metal foil conductive members, thefifth contacting the first face of the first PTC element and spacedapart from the first external electrode, and the sixth contacting thefirst face of the second PTC element and spaced apart from the secondexternal electrode. Generally there are seventh and eighth residualinternal metal foil conductive members, the seventh contacting thesecond face of the first PTC element and spaced apart from the thirdinternal electrode, and the eighth contacting the second face of thesecond PTC element and spaced apart from the fourth internal electrode.The device may also comprise one or more additional laminar polymerelements, which may be conductive or insulating. Preferably one of theadditional elements is a third laminar polymer element which isinsulating, lies between the first and the second PTC elements, and issecured to the exposed internal surfaces of the PTC elements which maycomprise the internal faces of the PTC elements or their internalelectrodes or internal conductive members. The PTC elements, theelectrodes and the residual conductive members define two apertures, thefirst of which runs between the first external electrode, the seventhand eighth internal residual conductive members, and the second externalelectrode, and the second of which runs between the fifth residualexternal conductive member, the third and fourth internal electrodes andthe sixth residual external conductive member, and through the first andsecond PTC elements and the third laminar polymer layer, if present. Inaddition, the device comprises first and second transverse conductivemembers which are composed of metal. The first transverse conductivemember lies within the first aperture, and is physically andelectrically connected to the first and second external electrodes andthe seventh and eighth internal residual conductive members. The secondtransverse conductive member lies within the second aperture, and isphysically and electrically connected to the fifth and sixth externalresidual conductive members and the third and fourth internalelectrodes.

Other embodiments of the device may not contain residual (also calledadditional) conductive members.

The devices of the invention can be of any appropriate size. However, itis an important advantage for applications to make the devices as smallas possible. Preferred devices have a maximum dimension of at most 12mm, preferably at most 7 mm, and/or a surface area of at most 60 mm²,preferably at most 40 mm², especially at most 30 mm², and the surfacearea can be much smaller, e.g. at most 15 mm².

Processes

The methods disclosed herein make it possible to prepare devices veryeconomically by carrying out all or most of the process steps on a largestack of laminates, and then dividing the laminate into a plurality ofindividual composite devices. The division of the stack can be carriedout along lines which pass through any, some or all of the conductivesurfaces, or through any, some or all of the cross conductors. Theselines of division, also called isolation lines or delineation lines, canbe of any shape suitable for producing devices of a particularconfiguration, e.g. straight, curved, or at angles. Similarly“functional” lines, e.g. the gaps between an electrode and a residualmember, can also be of any suitable shape. The process steps prior tothe division can in general be carried out in any convenient sequence.For example, it is often convenient to pattern the internal conductivesurfaces prior to assembling the stack, and to pattern the externalconductive surfaces after assembly. However, it is possible to patternboth internal and external conductive surfaces prior to assembly. Thepatterning of a conductive surface can be the same as or different fromthat on other conductive surfaces in the stack, depending on the desiredfunctionality of the final device. For example, FIGS. 5, 6, 8, 9, 11 and12 show devices which have internal electrodes which are the mirrorimages of external electrodes. FIGS. 10 and 18 to 20 show devices whichhave internal electrodes which are patterned differently than externalelectrodes. Often, it is useful to pattern conductive surfaces byremoving, e.g. by etching, stamping, or milling, conductive material.Alternatively, the pattern can be produced by an additive process, e.g.screen printing, sputtering, or deposition. For some applications, it isuseful to remove strips of conductive material in staggered stripsalternately from opposite sides of a laminate layer to balance thephysical stresses in the product. The resulting pattern will containgaps or recesses suitable for separating a second electrode from aresidual member in a device, separating one device from another,providing delineation for subdividing the assembly into individualdevices, allowing for orientation of individual laminates or theassembled stack, or providing marking.

Cross-conductors, i.e. electrical connections, can be formed before orafter the laminates are formed into the stack. If it is desired to formcross-conductors which do not traverse all layers of the stack, it maybe convenient to form them for the desired laminate layers only, andthen assemble the stack. Alternatively, using a blind via process, theconnection can be made after the stack is assembled. Assembly of thestack can be accomplished in stages, for example, some laminates areprepared and secured together, some further processing steps areperformed on the partially constructed stack (e.g. formation and platingof cross-conductors), and then other laminates can be secured to thispartially constructed stack to complete the assembly. Subdivision of thestack into composite devices can be accomplished using a variety oftechniques such as sawing, shearing, dicing, punching, and snapping,e.g. by using a saw, a shear, a blade, a wire, a waterjet, a snappingdevice, a laser, or a combination of these. Some preferred processes formaking devices from a single laminate are disclosed in U.S. Pat. No.5,864,281. These processes may be adapted for the subdivision of a stackof laminates such as that described herein. Alternatively, some of thesteps in the process, e.g. securing the laminates in a stack and makinga plurality of electrical connections by means of cross-conductors, canbe accomplished simultaneously.

In order to minimize curling or warping due to shrinkage duringsubsequent processing steps, it may be preferred to apply a patternaround the perimeter of at least one laminate. The preferred pattern isproduced by a process comprising selectively removing conductivematerial from at least one conductive surface of each laminate in analternating cross-directional design, e.g. in the shape of a “W” or a“Z”, around the perimeter of the laminate such that there is electricalcontinuity between the outer edge of the conductive surface for theexternal layers.

Part or all of the external conductive surface may be covered with aninsulating layer, e.g. a solder mask or marking material such as thatdisclosed in U.S. Pat. No. 5,831,510.

The invention is illustrated in the accompanying drawings, in whichfeatures such as apertures and the thicknesses of components are notdrawn to scale to make them more clear. FIG. 1 is a perspective view ofa section of a stack 1 which has two laminar elements 7 and 8, each ofwhich has patterned external conductive surfaces 3 and 3′, respectively,and patterned internal conductive surfaces 5 and 5′, respectively. Theelements 7 and 8 are secured to each other with insulating laminarelement 6. Tubular cross conductors 11 extend through the stack asshown.

FIG. 2 is an exploded view of a stack under construction according tothe process of the invention. Two laminar elements 7 and 8, each havingpatterned internal conductive surfaces 5 and 5′, respectively, andunpatterned external conductive surfaces 3 and 3′, respectively, areincluded in the stack, with a laminar element 6 sandwiched between 7 and8. Registration holes 4 are used to orient the elements of the stackuniquely, align them relative to each other, and to position the stackfor subsequent processes such as patterning of external surfaces and theformation of apertures.

FIG. 3 is a plan view of a patterned external conductive surface 3 of asection of a stack. C marks where the subdivision of the stack intocomposite devices will occur. FIG. 4 is a cross-sectional view alongline IV-IV of FIG. 3. The stack includes laminar elements 7 and 8 eachhaving internal conductive surfaces 5 and 5′ and external conductivesurfaces 3 and 3′, and laminar element 6 which is sandwiched between 7and 8. The stack has been plated to provide the tubular cross conductors11 in each of the apertures (and a plating 12 on other exposed externalsurfaces of the stack). The stack as shown will be subdivided throughthe tubular cross conductors, forming cross conductors with semicircularcross-sections.

FIG. 5 is a perspective of a composite device 2 formed by subdividing astack. Two laminar PTC elements 17 and 18, each having externalelectrodes 14 and 14′, respectively, external residual conductivemembers 36 and 36′, internal electrodes 16 and 16′ and internal residualconductive members 38 and 38′, are secured together with laminar element26. A first transverse member 31 and a second transverse member 51 arehollow tubes formed by a plating process in which the exposed surfacesare plated with copper and then with solder to form a first plating 32on transverse member 31, and a second plating 52 on transverse member51. A dielectric coating 55 covers the external surfaces of the deviceexcept where it is desired to make electrical connection. A plating 12is applied to the exposed portions of the external electrodes. Theregion between the dashed lines indicates portions under the dielectriccoating 55 in which there is no electrode material.

FIG. 6 shows in cross-section a composite device 2 as in FIG. 5 solderedto traces 41 and 43 on an in insulating substrate 9.

FIG. 7 is a plan view for a variety of composite devices shown incross-section along line VIII-VIII in FIGS. 8 to 11, 14, and 21. Thedashed lines indicate portions positioned underneath the dielectriclayer 55, wherein the electrode material is not present. Note that forthe cross-sectional views in FIGS. 8-11, 14 and 21, the dielectric layer55 is not shown. FIGS. 8 and 9 show two configurations for PTC elementsconnected in parallel. For the device shown in FIG. 8 when it is in itsswitched, high resistance state, the laminar element 26 will have nopotential drop across it when a potential is applied to one externalelectrode 14 and one external residual conductive member 36. However,for the device shown in FIG. 9, when in its switched state, the laminarelement 26 will have a potential drop across it for the same externalelectrical connection. FIG. 10 shows a variant of the device shown inFIG. 8, with no internal residual conductive members. FIG. 11 shows acomposite device formed by connecting three laminar elements 17, 18, and19 in parallel, with laminar element 26 between 17 and 18, and laminarelement 26′ between 18 and 19. The version of the device shown hasinternal electrodes 16, 16′, 16″ and 16′″, and internal residualconductive members 38, 38′, 38″, and 38′″.

FIG. 12 is a plan view of a device with no residual conductive members;a cross-sectional view along XIII-XIII is shown in FIG. 13. Dashed linesindicate regions positioned under the external dielectric layer 55 whereelectrode material is not present. Dielectric layer 55 is not shown inFIG. 13.

FIG. 14 is a composite device formed by the process of this invention,in which cross conductors do not extend completely through all layers ofthe stack. To make the device as shown here, cross conductors 59 extendonly between external and internal conductive surfaces of each laminarelement of the stack; the laminar elements are then secured togetherusing a anisotropically conductive substance 57 which conducts only inthe z-direction, where z- refers to the direction from bottom to top ofthe composite device. Conductive substance 57 provides electricalconnection between internal residual conductive members 38 and 38′ andbetween internal electrodes 16 and 16′, without causing 38 or 38′ to beshorted to 16 or 16′.

FIG. 15 shows a composite device in which laminar elements 17 and 18 areconnected in series. The laminar elements are secured together in thestack using a conductive material 61. Cross-conductors are made in thestack which connect to some conductive surfaces, but not all. To formsuch a cross conductor, apertures larger than the desired dimensions ofthe cross conductor are formed through the stack. The apertures are thenfilled with an insulating substance 63, and two smaller apertures 65 and67 are formed within the volumes filled with the insulator 63. Theapertures 65 and 67 and exposed external electrodes have platings 32 and52.

FIG. 16 shows a plan view of a composite device which includes twodevices and has three external electrical connection points. A diagramof the electrical connections for the two devices 77 and 79 is shown inFIG. 17.

FIG. 18 shows a cross-section along line XVIII-XVIII in FIG. 16.Cross-conductor 52 is in electrical contact with the internal residualmembers 38 and 38′. A gap separates the residual members 38 and 38′ frominternal electrodes 16 and 16′. Additional conductive member 46′ is alsopresent.

FIG. 19 shows a cross-section along line XIX-XIX in FIG. 16.Cross-conductor 72 is in electrical contact with internal residualconductive members 38 and 38′. A gap separates the residual members 38and 38′ from internal electrodes 16 and 16′. Additional conductivemember 46 is also present.

FIG. 20 shows the cross-sectional view along XX-XX in FIG. 16.

FIG. 21 illustrates a composite device which has only one internalelectrode 16, formed from a stack with only one internal conductivesurface. A laminar element 17 is combined with the laminar element 78.The laminar elements can be pressed together to form a bond, so that nothird laminar element is required to secure the laminar elementstogether. For example, 17 can comprise a PTC element and 78 can comprisea dielectric substrate with adhesive properties.

FIG. 22 is a plan view of a composite device which has multiple crossconductors to provide extra current carrying capacity, and extrarobustness should one or both of the other cross conductors becomedamaged or form an open circuit. The dashed line indicates regions wherethere is no electrode material; the dotted circle indicates the regionof an additional cross-conductor.

FIG. 23 is a cross-sectional view along XXIII-XXIII in FIG. 22(dielectric layer 55 not shown). A third aperture 81 has a metal plating82 and forms an additional electrical connection between the internalelectrodes 16 and 16′. Note that there is a region around the externalelectrodes 14 and 14′ where no electrode material is present.

The invention is illustrated by the following Example.

EXAMPLE

A stack in accordance with FIGS. 1 and 2 was prepared by the followingmethod. Two laminates, each having a thickness of about 0.264 mm (0.0104inch) were prepared by attaching a nickel/copper foil having a thicknessof about 0.0356 mm (0.0014 inch) to both sides of a 0.193 mm (0.0076inch)-thick sheet of conductive polymer. The conductive polymer wasprepared by mixing about 40% by volume carbon black (Raven™ 430,available from Columbian Chemicals) with about 60% by volume highdensity polyethylene (Chevron™ 9659, available from Chevron), and thenextruding into sheet and laminating in a continuous process. Thelaminated sheet was cut into individual laminates of 0.30 m×0.41 m (12inch×16 inch). The laminates were irradiated to 4.5 Mrad using a 4.5 MeVelectron beam.

Each of the laminates was drilled in an asymmetric pattern around itsthe periphery to provide holes and slots to register the laminates in aknown x-y orientation in the plane of the laminate. These registrationholes and slots were used to align plaques relative to each other informing a stack, and for subsequent alignment of the tooling forimaging, solder masking, and plating operations. A 0.0762 mm (0.003inch)-thick layer of modified acrylic adhesive (Pyralux™ LFO, availablefrom DuPont) was also drilled with registration holes suitable foralignment.

One surface of one foil layer of each of two laminates were patternedusing an etching technique in which they were first coated with an etchresist, then imaged in a desired pattern. The etch resist was developedand etching was accomplished using cupric chloride before the resist wasstripped away. These same foil layers were patterned to define theperiphery of the individual devices and the residual conductive members.In addition, the outer edges of the metal foil on the laminate wereetched to provide an alternating cross directional pattern around theperimeter, as shown in FIG. 2. Paths providing electrical continuitywere utilized during the subsequent electrolytic plating of Sn/Pb.

A stack was formed by positioning two laminates with theirpattern-matched etched sides facing inward, and with an adhesive layersandwiched in between, as shown in FIG. 2. A fixture was used to alignthe layered laminates, and the stack was heated while under pressure topermanently attach the layers into a laminated structure. The thicknessof the stack formed was approximately 0.61 mm (0.024 inch).

Holes having a diameter of 0.94 mm (0.037 inch) were drilled through theentire stack to form apertures. The stack was treated with a plasmaetch. The apertures were then coated with colloidal graphite, and thestack was electrolytically plated with copper.

The external metal foil layers of the stack were then patterned byetching. The registration holes were used to ensure the pattern whichwas etched was properly aligned with the internal layers which wereetched previously. An alternating cross-directional pattern around theedges was etched as previously described.

A soldermask (Finedel DSR 2200 C-7, available from Tamura Kaken Co.,Ltd.) was applied to one external metal foil layer of the stack,tack-cured, and then applied to the second external metal foil layer ofthe stack, and tack-cured. The soldermask was then imaged and developed.Marks were applied to identify individual parts, and the panel was thenheated to fully cure the mask. A SnPb solder plate was deposited in thesolder pad regions for use in attaching devices to circuit boards.

The assembly was divided to produce devices as shown in FIG. 5 by firstseparating the assembly into strips using a shear or saw, and thensubdividing the strips into individual devices by mechanical snappingusing a two-step process in which the strips were first bent to form afracture in the conductive polymer at the isolation line, and then weresheared along that isolation line. The devices produced had dimensionsof about 4.5 mm×3.4 mm×0.7 mm (0.179 inch×0.133 inch×0.029 inch), and aresistance of about 0.031 ohm. Following installation onto a printedcircuit board by solder reflow, the devices had a resistance of about0.050 ohm.

1. A process for manufacturing a composite polymeric circuit protectiondevice, said process comprising (1) providing a polymeric assemblycomprising (a) providing first and second laminates, each of whichcomprises a laminar polymer element having two conductive surfaces, (b)providing a pattern of conductive material on at least one of theconductive surfaces on one laminate; (c) securing the laminates in astack in a desired configuration by means of an adhesive, one conductivesurface of each of the first and second laminates comprising an externalconductive surface of the stack, and (d) making a plurality ofelectrical connections between a conductive surface of the firstlaminate and a conductive surface of the second laminate; and (2)subdividing the assembly into individual devices each of which comprisesat least one electrical connection.
 2. The process according to claim 1,wherein the pattern in step (b) is formed by selectively removing aportion of conductive material from at least one of the conductivesurfaces on one laminate.
 3. The process according to claim 2, whereinthe selective removal of conductive material is accomplished by etching,milling, or stamping.
 4. The process according to claim 1, which furthercomprises providing a pattern of conductive material on at least one ofthe external conductive surfaces.
 5. The process according to claim 4,wherein the pattern on the external conductive surface is formed byselectively removing a portion of conductive material from the externalconductive surface.
 6. The process according to claim 4, wherein atleast one of the patterned external conductive surfaces is at leastpartially covered with an insulating layer.
 7. The process according toclaim 4, wherein the patterns on the internal and external conductivesurfaces are different.
 8. The process according to claim 1, wherein anadditional conductive layer is added to at least part of at least one ofthe external conductive surfaces.
 9. The process according to claim 1,wherein at least one laminate is marked to provide a uniqueidentification of orientation.
 10. The process according to claim 9,wherein the laminate markings which provide a unique identification oforientation also provide delineation for subdividing into individualdevices.
 11. The process according to claim 1, wherein the assemblycomprises a third laminate.
 12. The process according to claim 11wherein the third laminate comprises a laminar polymer element havingtwo conductive surfaces.
 13. The process according to claim 1, whereinelectrical connection is made between conductive surfaces of the firstand second laminates in the stack by (i) forming an aperture whichextends through the stack, and (ii) forming a conductive member withinthe aperture.
 14. The process according to claim 1, wherein theelectrical connections are positioned so that the individual devicecomprises at least two electrical connections.
 15. The process accordingto claim 1, wherein the laminar polymer element in at least one of thelaminates comprises a PTC conductive polymer composition.
 16. Theprocess according to claim 15, wherein the laminar polymer element ineach laminate comprises the same PTC conductive polymer composition. 17.The process according to claim 15, wherein the laminar polymer elementin each laminate comprises a different PTC conductive polymercomposition.
 18. The process according to claim 1 wherein the assemblycomprises three laminates, each of which comprises a PTC conductivepolymer composition.
 19. The process according to claim 1, wherein atleast one of the laminar polymeric elements comprises a ZTC conductivepolymeric material or an NTC conductive polymeric material.
 20. Theprocess according to claim 1, wherein at least one of the laminarpolymeric elements comprises an insulating polymeric material.
 21. Theprocess according to claim 1, wherein the individual devices aresubdivided from the assembly using a saw, a shear, a blade, a wire, awaterjet, a snapping device, a laser, or a combination of these.
 22. Theprocess according to claim 1 wherein the conductive surface on eachlaminate comprises a metal foil.